1. Field of the Invention
The present invention relates to semiconductor structures. More particularly, the present invention relates to metal interconnect structures having large grain sizes at a bottom of a metal interconnect line and methods of manufacturing the same.
2. Description of the Related Art
Current sub-90 nm copper interconnect technology has a non-bamboo microstructure, that is, a microstructure without bamboo-like features in lines and vias. The non-bamboo microstructure leads to concerns associated with copper diffusion such as electromigration and stress voiding.
There are currently three different modes of copper diffusion. One mode is copper diffusion along grain boundaries of a copper interconnect structure. Another mode is copper diffusion at a surface of a copper interconnect structure, that is, at an interface at which the copper interconnect structure adjoins another material. An alternate mode is copper bulk diffusion through grains, that is, across an interface at which two grain boundaries meet. Typically, the rate of diffusion is higher for copper diffusion along grain boundaries and lower for copper bulk diffusion through grains. Thus, it is optimal to form a copper interconnect structure containing a metal line in which the metal line has a bamboo-like pattern in the grain microstructure, or a “bamboo microstructure.” In a bamboo microstructure, the lateral width of a grain is the same as the width of the metal line or the metal via. The length of the grain is greater than the width of the metal line so that grain boundaries look like a stalk of a bamboo plant with notched segmentation.
It is optimal to have a bamboo microstructure where grains span the width and height of a line or via. The phenomenon of electromigration occurs when a current flowing in the line, due to an externally applied field, leads to a net drift of copper (Cu) ions in the direction of the electron flow. The drift eventually will lead to line failure due to loss of copper at divergent sites such as grain boundaries and material interfaces. Because electrical current flows along the direction of a metal line and any electromigration is forced to occur “through,” that is, substantially perpendicular to the plane of grain boundaries, the bamboo microstructure offers significantly more resistance to electromigration than non-bamboo microstructures. The bamboo microstructure essentially shuts down diffusion along grain boundaries, because bamboo grain boundaries are substantially at right angles to the current flow.
An alternative way of suppressing electromigration in a metal interconnect structure exists. If the length of a metal line is less than the “Blech” length, copper ion motion will not occur, shutting down the electromigration process. Mechanical stress at lengths less than the “Blech” length opposes the drift of copper ions. A typical Blech length is 10 microns for current interconnect structures consisting of copper. In principle, designing all interconnect metal lines shorter than the “Blech” length would solve the problem. In practice, such a limitation puts a severe constraint on the design and layout of an interconnect structure, and practically renders such layouts impractical.
In a related patent, U.S. Pat. No. 7,843,063, commonly assigned, it is disclosed that cobalt (Co) has a similar property, promoting normal grain growth (growth of all orientations simultaneously) or abnormal grain growth (growth of certain orientations preferential to others) in the fine lines and vias leading to bamboo grains (spanning the line width and height). Although, cobalt (Co) and manganese (Mn) have similar properties, Mn has a better optimal percentage over the use of Co.
The use of Cu—Mn seed layers has been contemplated in order to form “self-forming” diffusion barriers. The Mn is placed in the Cu seed layer and after thermal treatment diffuses to interfaces reacting with oxygen (O) to form manganese oxide (MnO) and possibly manganese silicate (MnSiO) layers. These layers at the dielectric-copper (Cu) interface or barrier-copper (Cu) interface act as diffusion barriers. Some publications that describe the use of MnO as a diffusion barrier are:    J. Koike et al., Appl. Phys. Lett. 87, (2005), 041911; J. Iijima et al., Proc. of IITC, (2006), 246; T. Watanabe et al., Proc. of IITC, (2007), 7; M. Haneda et al., Proc. of AMC (2207), 59.
Suppressing copper diffusion without resorting to use of a design rule stipulating that all metal interconnect lines be shorter than the “Blech” length is needed. Thus, there exists a need for metal interconnect structures having fine feature sizes such as sub-90 nm metal lines, i.e. metal lines having a width less than 90 nm, and containing bamboo microstructures so that copper diffusion and associated complications can be avoided. A bamboo grain, one spanning the width and height of an interconnect or via, every “Blech” length (10 μm), will substantially stop electromigration along grain boundaries.